Chap. XVI.] THE FRESH-WATER MUSSEL. 309 



At the anterior end of the pericardium is seen the brownish 

 tissue of the organ of Keber (k. o.). 



Alimentary System (Fig. 90, B.). For the dissection of the 

 alimentary canal it is well to work on a specimen that has lain 

 for two or three days in spirit. With the aid of the figure little 

 difficulty should be found in exposing the canal, and slitting it 

 open by passing a guarded bristle (tipped with sealing-wax), and 

 following it with scissors. 



The mouth, which lies between the anterior adductor and the 

 foot, leads by a short oesophagus into the so-called stomach (st.) ; 

 an irregular sac imbedded in the brownish tissue of the 

 digestive gland (d. gl.\ the secretion of which enters by ducts 

 (d. d. gl.) near its anterior end. From the stomach the intestine 

 passes downwards and takes the coiled course indicated in the 

 figure. Into its cavity (at ty.) there projects an infolding of the 

 wall or typhlosole. The intestine passes upwards and enters the 

 pericardium at its anterior end, running backwards thence as the 

 rectum (r.), which passes through the ventricle (ve.) of the heart. 

 The rectum, in which the typhlosole is well marked, leaves the 

 pericardium near the dorsal side of its posterior end, and passes 

 over the posterior adductor muscle and along the dorsal side of 

 the supra-branchial chamber, where it ends in an anus (v.) placed 

 on a prominent papilla. 



On scraping the lining membrane of the intestine, ciliated and 

 columnar cells will be found. Some of the columnar cells 

 would seem to be secreting cells. 



On teasing out a small fragment of the digestive gland, it is 

 seen to be composed of branched caecal tubes. Their lining 

 epithelial cells, examined under a high power, show, in addition 

 to the protoplasm and nucleus, a vesicle which encloses spherical 

 brownish-green granules. 



The food of the mussel consists of minute organisms swept 

 into the infra-branchial chamber by the inhalent current caused 

 by the lashing of the innumerable cilia on the gills. 



The Gill-Plates. The plate-like gills, which give to the class 



